The invention relates to air heaters of the type that are used in steam generation apparatus and more particularly to apparatus for partitioning the discharge of a bisector air heater into two or more zones.
Air heaters (also known as preheaters) are used in steam generation apparatus where they have two important functions. They cool the gases exiting from the generation apparatus before the gases pass to the atmosphere. Simultaneously, air heaters raise the temperature of the incoming air that is used in the combustion process within the steam generation apparatus. This has the effect of increasing fuel firing efficiency. Air heaters are necessary for pulverized coal firing to provide (a) drying of the coal and (b) a transporting medium. The present invention has particular application to rotary regenerative air heaters. This type of air heater transfers heat to a large drum-shaped rotor that turns continuously through the flue gas and incoming air (forced draft) air streams.
In a typical heater of this type, there is a vertical shaft on which the rotor turns. The heater typically is provided with shrouds to direct air flows in two axially extending cylindrical sections. In the bisector air preheater, the sectors are defined by a single vertical plane extending through the vertical shaft on which the rotor turns. Typically, the flue gases are directed downwardly in one cylindrical section and the incoming ambient (forced draft) air is directed through the other cylindrical section.
A typical rotor for such air preheaters has either 12 or 24 heat absorbing radial members attached to a vertical shaft. The rotor sealing system includes leaf type labyrinth seals that are used with axial sealing plates. This system effectively separates the incoming ambient air (forced draft) air stream from the flue gas stream. Both the radial and axial sealing plates may be adjusted during operation for maximum effectiveness.
Other heater constructions may accommodate horizontal flow. The vertical shaft air preheater can be arranged for flue gas flow upward or downward, while the horizontal air preheater can be arranged for gas-over-air or air-over-gas at the option of the plant designer. An electric motor drives the rotor through a speed reducer pinion gear which engages a pin rack attached to the periphery of the rotor. Such regenerative air heaters are typically designed with rotor diameters of between 7 feet and 65 feet.
A design compromise in conventional regenerative bisector air heaters in coal fired steam generation apparatus is that same temperature air is supplied to both the combustion cavity and the pulverizer(s) for the coal. In practice, the requirements for the pulverizer(s) and the combustion process are not the same. The prior art required a compromise between the competing requirements.
Conventionally, the air supplied to the pulverizer(s) is taken from the forced draft air duct well downstream of the air heater. Because the air is extracted from the duct at a point removed the air heater, the air is at substantially a uniform temperature. The utilization of the forced draft air taken downstream of the air heater is satisfactory for some applications. However, the lack of control of the temperature of the air supplied to the pulverizer is not satisfactory for many applications.
It is an object of the present invention to provide apparatus which will allow control of the air temperature flowing to the pulverizers.
It is another object of the invention to provide a structure that will allow the control of the temperature of the air supplied to the pulverizers.
Another object of the invention is to provide apparatus which will be relatively simple in construction.